Feature request: Automatic test.done() call in asynchronous tests

Nodeunit has great feature in expect, but ending asynchronous tests is often painful, mainly in tests with more than 1 callback. I think, it has easy solution and implementation isn't hard.

Instread of passing raw callback, pass returned callback from `test.callback([expect 1,] callback);`.

Invoking `test.callback` will store given callback and mark him as done after expected number of calls (internal mechanism will watch status of each callback). Each call will invoke original callback. If all test.callback are marked as done, test.done is called automatically.

It should handle all cases, even callback in callback.

It doesn't break existing code, it is optional.

Implementation could look like this:

``` javascript
test.callback = function(expected, callback) {
if (typeof expected === 'function') {
callback = expected;
expected = 1;
}

this.callbacks = this.callbacks || 0;
this.callbacks++;

var test = this;
var done = false;

return function() {
expected--;
if (done===false && expected<=0) {
done = true;
test.callbacks--;
}

var ret = callback.apply(this, arguments);

if (test.callbacks<=0) {
test.done();
}

return ret;
};
}
```

Beginne damit, die asynchrone Testverarbeitung und die bestehenden Einstiegspunkte test.callback und test.done zu lokalisieren. Verfolge, wie Callbacks derzeit registriert und abgeschlossen werden, und prüfe anschließend die vorgeschlagenen Fälle mit mehreren Callbacks und verschachtelten Callbacks. Als abgeschlossen gilt dies, wenn der optionale Callback-Wrapper das ursprüngliche Callback-Verhalten beibehält und test.done erst aufruft, nachdem alle erwarteten Callbacks abgeschlossen sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.